In a fuel injection system in which fuel is supplied from a common rail (pressure accumulator) to a fuel injector, when the fuel injector injects the fuel, a fuel pressure in the fuel injector varies according to an injection rate variation. A fuel pressure sensor detects a variation waveform of the fuel pressure when the fuel is injected. Based on a detected variation waveform of the fuel pressure, a waveform indicating the injection rate variation is estimated.
JP 2017-53309 A shows that a moving average of fuel pressure differential values is calculated and a fuel pressure characteristic is analyzed based on a fuel pressure waveform of the fuel pressure identified by the moving average. Specifically, a maximum value and a minimum value of the moving average are detected, and an approximate straight line is calculated by least squares method. Intersections of the approximate straight line and a reference straight line are defined as an injection start timing and an injection end timing.
It has been confirmed that a water hammer wave (pressure wave) is generated due to a start of fuel injection, and such water hammer wave affects a detection of an injection end timing. According to the technique shown in JP 2017-53309 A, it is likely that an injection end timing may be erroneously detected.
Especially, in a common rail, it is conceivable that the influence of the water hammer wave depends on manufacturing tolerance. A fuel pipe is connected to the common rail through an orifice. An orifice diameter has a variation due to manufacturing tolerance. The variation in orifice diameter affects a fuel pressure variation which is generated due to the water hammer wave, so that an injection end timing may be erroneously detected.